Add justification for extrapolation

Author: nicktindall

x-pack/plugin/write-load-forecaster/src/main/java/org/elasticsearch/xpack/writeloadforecaster/LicensedWriteLoadForecaster.java

@@ -168,7 +168,9 @@ static OptionalDouble forecastIndexWriteLoad(List<IndexWriteLoad> indicesWriteLo
             // any write-load data beyond their lifetime.
             // To avoid making assumptions about periods for which we have no data, we'll weight
             // each index's contribution to the forecast by the maximum shard uptime observed in
-            // that index.
+            // that index. It should be safe to extrapolate our weighted average out to the
+            // maximum uptime observed, based on the assumption that write-load is roughly
+            // evenly distributed across shards of a datastream index.
             allIndicesWriteLoad += totalIndexWriteLoad * maxShardUptimeInMillis;
             allIndicesUptime += maxShardUptimeInMillis;
         }